The present invention is in a process for producing a phosphate coating on a metal having a surface which consists at least in part of iron or steel by a treatment with an aqueous phosphatizing solution, which contains zinc ions, manganese ions, PO.sub.4 ions, nitrate ions and if being in working condition, iron(II) ions and to the use of that process for preparing metals for subsequent cold working.
It is known to phosphatize metals, particularly iron and steel, by a treatment with phosphatizing solutions which contain manganese, iron(II) and zinc phosphate and are at temperatures of from 50.degree. to 98.degree. C. The phosphate coatings thus obtained are suitable for all fields of application which are known in phosphatizing technology, such as rust prevention, priming for the application of paint, electric insulation, reducing sliding friction and facilitating cold working (Published German Application No. 30 23 479; EP No. 42 631).
The performance of the known processes at relatively low temperatures, e.g., between 30.degree. and 50.degree. C., often gives rise to difficulties which reside, among others, in the formation of incompletely covering phosphate coatings which become progressively coarser as the iron(II) content in the phosphatizing solution increases. Certain improvements can be obtained by a preliminary activating rinsing step, e.g., with titanium phosphate, but the results thus produced are not entirely satisfactory. On the other hand, low-temperature processes are urgently required because they save energy. Additionally, the processes which involve the presence of iron(II) are superior to the use of phosphatizing solutions which are free of iron(II) because much less sludge is formed and the consumption rates are favorable.